I'm sorry to hear you got bitten by that. It has happened to me too, even though I usually compose replies in TextEdit and copy/paste to the post entry box. That practice has saved my bacon far more often, but even so #*%@ happens...
Anyway, while I'm not sure this particular message can be changed by us (i.e., users of the software), I wonder what the solution might be. What changes would you suggest for this particular message that would have stopped you from being bitten? Please also consider that this message may occur in different situations, and that a more specific wording may not be appropriate for all instances it's currently called for.
I've lost posts for assorted reasons over the years, and I not only frequently compose in TextEdit, I frequently forget to do so.
It's difficult to suggest an alternative, because I don't know what would have happened had I simply clicked command-back arrow (or left bracket) or menu bar > back arrow, but if those would have taken me back to my response it would seem to be the obvious language.
The existing language is fine when, for instance, you hit "Notify" and an "...already...." page comes up, although my alternative language would serve the same purpose, i.e. no conflict.